In modern wireless communication systems it may be desirable for a transmitter or transceiver to operate simultaneously in both an efficient and linear manner. It also may be desirable for the transmitter to operate in multiple frequency bands. Power amplifiers used in transmitters may be optimized for use in a particular mode and frequency band to maximize efficiency. Such optimization may require the amplifier to be biased in a certain manner. Additionally, impedances may need to be matched between components within the power amplifier and between the amplifier and adjacent components.
For example, a transmitter may be designed to operate in two separate frequency bands, such as the GSM850/900 (824-915 MHz) and DCS1800/PCS1900 (1710-1910 MHz) frequency bands, or the CDMA800 (824-849 MHz) and CDMA1900 (1850-1910 MHz) frequency bands. Impedance may be dependant on the operating frequency and, therefore, a power amplifier having optimal impedance matching in one frequency band may not be optimized for operation in a different frequency band. Problems related to impedance matching at different frequencies may be solved by providing separate amplifying chains. However, separate amplifying chains may require numerous switches to provide a desired combination of amplifier stages. This may make it difficult to implement the amplifier in a monolithic integrated circuit design. It also may increase the size and the power requirements for the transmitter.
In addition, many wireless communications systems, such as GSM/EDGE, CDMA2000, or WCDMA, may require that the power amplifier be capable of delivering a wide range of output powers. There may be a tradeoff between efficiency and linearity, however, with improvement in one coming at the expense of the other. As a result, being designed for the highest power level with maximum available efficiency, a power amplifier may tend to operate less efficiently at lower power levels, which may shorten the life of a battery and reduce talk time duration.
Several approaches may be used in trying to solve these problems. For example, a dual-band or quad-band mobile phone transmitter may contain two power amplifiers, or a power amplifier module including two separate amplifier chains, each operating in a single frequency band. One problem with this approach, however, is poor efficiency at backoff output powers (i.e., output powers less than the maximum power). This is because amplifiers typically are designed to provide maximum efficiency at maximum output power. In addition, a high-efficiency broadband power amplifier that can operate in a variety of desired frequency bands simultaneously also may require different matching circuitry for different frequencies and/or device impedances.
One system employs a dual-band single-stage power amplifier for operation in either the 800 MHz band or the 1900 MHz band using the same active amplifier device, with different switching impedance networks at both the input and the output, to provide desired input and output impedances for operation in both frequency bands. This approach, however, requires an increase in the number of required switches and impedance networks, which may increase both the size and the power requirements of the power amplifier and the transmitter. In addition, the problem of poor efficiency at backoff power levels remains.
Another system uses a multi-stage power amplifier with bypass switches between stages for selectively bypassing one or more of the amplifier stages. This approach, however, requires switches between amplifier stages, as well as separate input and output matching networks for each stage. These requirements may make it difficult to implement the amplifier in a monolithic integrated circuit design, which may result in increased size and cost of the entire power amplifier and transmitter. In addition, for dual-band or quad-band power amplifier modules, this approach may require entirely separate chains of amplifier stages, switches, and impedance matching circuits for the additional frequency bands.
Accordingly, there is a need for a simple and efficient power amplification system. There is a further need for an efficient amplification system with a simplified implementation that requires a reduced number of external switches and/or impedance matching circuits. There is a further need for an efficient amplification system that is capable of operating in various frequency bands.